1. Field of the Invention
The invention relates to an image composition method comprising the steps of making a series of consecutive sub-images each representing a portion of an elongate scene, which sub-images overlap along the elongate scene, and of merging the sub-images into an assembled image. The invention also relates to an image processor for performing said method. The invention further relates to an x-ray examination apparatus comprising an image processor for performing said method.
2. Description of the Related Art
An image composition method of said kind and a device for performing this method are known from the United States Patent U.S. Pat. No. 5,123,056.
The cited reference describes, in particular, an image composition method for use in peripheral angiography by means of x-ray radiography. The x-ray examination apparatus described in the cited reference comprises an x-ray source, an x-ray detector facing the x-ray source and a patient table. The patient table on the one hand, and the x-ray source and the x-ray detector on the other hand, are displaceable with respect to one another. Consecutive sub-images of adjacent portions of a patient's limb are made by successive irradiations while the patient is translated vis-a-vis the x-ray source over some distance between successive irradiations. X-ray imaging is performed on the patient's limb in peripheral angiography. Because of the size of the limb of interest, an x-ray examination apparatus is unable to image the entire limb at once during a single irradiation step. Instead, a number of x-ray sub-images are made of a patient's limb that is to be examined. These x-ray sub-images are converted into electronic sub-images by means of an x-ray image detector in the form of an image intensifier television chain that is included in the x-ray examination apparatus. In order to form a single image of the entire limb, the electronic sub-images are processed in such a way that overlapping portions of said electronic sub-images are deleted. Subsequently, from said processed electronic sub-images, an assembled image is formed by adjoining consecutive processed electronic sub-images. The assembled image is supplied in the form of an electronic videosignal which may be supplied to a monitor for direct viewing or to a device for forming a hard-copy of the assembled image.
In the image composition method of the cited reference the overlapping portions of consecutive sub-images are calculated from positions of a carrier to which the x-ray source and the x-ray detector are mounted, and from positions of the patient table of the x-ray apparatus during x-irradiation for the formation of each of the x-ray sub-images. Thus, the known image assembly method can be employed only if data on the positions of the x-ray source, x-ray detector and patient table are recorded together with the x-ray sub-images. It is common practice to fit an x-ray examination apparatus with a device for measuring and recording such positions. Consequently the known method suffers from a substantial drawback in that the known method is unsuitable for forming an assembled image from sub-images produced by an x-ray examination apparatus which does not record positions of the carrier and the patient table. In each of the sub-images, moreover, image-distortions arise, such as pincushion-distortion caused by the curvature of the entrance screen of the image intensifier, S-distortion caused by stray magnetic fields which perturb the electron-optics of the image intensifier, vignetting caused by to the spatial variations across the x-ray beam, and due to differences in the size of the patient's body. Furthermore, structures inside the patients' limbs, in particular bones, arteries and veins, are imaged with a parallax which varies among the sub-images. Consequently, the assembled image formed by the known assembly method contains disturbing transitions at the boundaries of the processed sub-images, as well as substantial pin-cushion distortions which vary over the entire assembled image. These image distortions have a deleterious effect on the diagnostic quality of the assembled image made by the conventional assembly method.